The present invention relates to ceramic compositions containing BaTiO.sub.3 as the basic composition, and particularly to such ceramic compositions having a high dielectric constant and in which the dielectric constant remains substantially stable despite temperature variation and the passage of time.
Ceramic compositions having the above described properties are suitable for use in various electronic devices, such as for instance ceramic capacitors. In order to be practical for use in ceramic capacitors, it is desirable that a material have a dielectric constant (.epsilon.) of up to 2,000. The dielectric constant should remain relatively stable over a wide temperature range, for example, from minus 55.degree. C to plus 125.degree. C and should have good stability over the passage of time. It is also desirable that the dielectric loss (tan.delta.) of the material be low.
In order to fulfill these requirements, a variety of compositions have heretofore been prepared. Most of these prior art compositions, however, have only had a dielectric constant which was stable with reference to either temperature or time. Moreover, the temperature range of good dielectric constant stability has been very narrow, so that practical applications have been quite limited.
It has been recently found that ceramic compositions containing barium titanate (BaTiO.sub.3) as the basic component and Ce, La and Bi as additives have a good temperature stability of their dielectric constant over a temperature range of -55.degree. C to +125.degree. C. The stability of the dielectric constant of such materials is also good with respect to time. This fact is reported in detail, for example, by Johannes Just, in an article entitled "Eigenschaften von hochtemperaturbestandigen Sonderkeramiken mit hoher Dielektrizitatskonstante," which appeared in `Technische Mitteilungen AEG-Telefunken Gesellschaft`, Vol. 60, No. 2, pages 125-126 (1970). However, in compositions of the type described in this article having dielectric constants which have good stability with respect to time and temperature, the dielectric constant is degraded to below 1,000 in value. Moreover, since Bi which has a high vapor pressure and is very reactive is contained in these compositions, it is difficult to use them in the manufacture of ceramics. When for instance these compositions are used to manufacture a laminated ceramic capacitor, internal electrodes made of platinum or palladium are noticeably corroded.
An object of the present invention is to eliminate the disadvantages mentioned above.
Another object of the present invention is to provide ceramic compositions which have a high dielectric constant and exhibit good temperature stability over a wide temperature range together with good time stability.
A further object of the present invention is to provide ceramic compositions which contain BaTiO.sub.3 as a basic composition but contain no Bi, so that capacitor manufacture using these compositions is facilitated and these compositions provide an optimum material for the manufacture of, for instance, laminated ceramic capacitors.